The key to the finding lies in an unusual genetic arrangement. In birds, females are the heterogametic sex, carrying one Z and one W chromosome, and both the W chromosome and mitochondrial DNA are inherited exclusively through the maternal line. Because these two genomes never recombine and travel together down the same maternal lineage, selection acting on any maternally inherited trait can drag linked mutations along with it—a process known as genetic hitchhiking.

The researchers found that this sustained selection subjects both the W chromosome and the mtDNA to increased accumulation of slightly deleterious mutations. The consequence is an accelerated pace of W chromosome pseudogenization and gene loss compared with related songbirds, in which egg colouration is not subject to the same intense selective pressure. In effect, the adaptation that helps cuckoo finch chicks survive in host nests carries a genomic cost, paid in the currency of decaying genes on the non-recombining W chromosome.

One result of the study stands out as particularly striking. As other W-linked genes were lost over evolutionary time, two genes involved in egg colouration—plausibly targets of selection during the coevolutionary battle between cuckoo finches and their hosts—were recruited to the W chromosome. Their relocation to the maternally inherited chromosome places them in the very genomic compartment where selection on egg appearance is most effectively expressed, since only females lay eggs and only maternal alleles are transmitted with the W.

The study also extends its analysis beyond the sex chromosome to the interaction between the mitochondrial genome and the nuclear genes that support it. Mitochondrial and nuclear genes must work together in the oxidative phosphorylation (OXPHOS) pathway, the molecular machinery that generates cellular energy, and mismatches between the two can harm fitness and even contribute to speciation. Using structural models of the respiratory complexes and the mitoribosome, the authors showed that the rate of amino acid substitution in nuclear-encoded residues is a function of their physical proximity to mtDNA-encoded residues. Nuclear genes whose products sit closest to mitochondrial components evolve fastest, a pattern the researchers interpret as evidence of compensatory mitonuclear coevolution, in which nuclear changes offset deleterious substitutions accumulating in the mitochondrial genome.

Together, these results connect a species-level ecological interaction—brood parasitism and host coevolution—to two genomic processes usually studied in isolation: sex chromosome degeneration and mitonuclear coevolution. The cuckoo finch’s dependence on maternal inheritance for its deceptive egg phenotype means that adaptation proceeds without recombination, and the study’s broader message is that such recombination-free adaptation leaves a distinctive genomic signature: hitchhiking of slightly deleterious mutations, accelerated gene loss on the W chromosome, and compensatory evolution in the energy-producing machinery of the cell.

The findings also carry a general caution for evolutionary genomics: traits inherited through a single parental line, whether sex chromosomes, mitochondrial DNA or other non-recombining regions, may bear hidden costs of adaptation that only become visible when compared across species with different selective histories.

For the cuckoo finch, the trade-off is clear. The same maternal inheritance that allows mimetic egg colouration to be selected so effectively in females also couples that trait to a W chromosome and a mitochondrial genome that cannot recombine to purge damage. The result, documented in this study, is a genome in which the machinery of deception and the machinery of cellular respiration have been reshaped by the same evolutionary force.

Subject of Research: Genomic consequences of maternal inheritance and genetic hitchhiking in the brood parasitic cuckoo finch

Article Title: Maternal inheritance accelerates sex chromosome degeneration and mitonuclear coevolution in a brood parasitic bird

Article References: Sorenson, M. D., Osipova, E., Spottiswoode, C. N., Jamie, G. A., Dixit, T., DaCosta, J. M., Sackton, T. B., Warren, W. C., & Balakrishnan, C. N. (2026). Maternal inheritance accelerates sex chromosome degeneration and mitonuclear coevolution in a brood parasitic bird. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03216-w

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03216-w

Keywords: cuckoo finch, brood parasitism, W chromosome, mitochondrial DNA, genetic hitchhiking, mitonuclear coevolution, egg colour mimicry, sex chromosome degeneration, comparative genomics, evolutionary biology, Maternal, inheritance